This proposed research aims to investigate on a subcellular level the autonomic, cholinergic innervation of the heart. The innervation of the heart is interesting because it differs in curious and probably revealing ways from the neuromuscular junction, which is often considered to be a model system for synaptic transmission. In the heart, the postganglionic axons terminate in a series of varicosities which are thought to be "en passage" sites of transmitter release. Varicose axons sometimes pass near the muscle fibers but often the varicosities are separated from the nearest target by several micrometers. Whether these distant varicosities actually release transmitter and whether the muscle responds to acetylcholine released from a distance is unknown. Further, since the heart does not exhibit postsynaptic membrane specializations characteristic of many focal synapses in the nervous system, the receptive membrane for transmitter has not been located. Finally, the response of the heart to acetylcholine has several interesting features, namely a long latency and slow timecourse, the mechanisms of which have not been characterized. This research will investigate the cholinergic receptors in the heart using intracellular recording and electrophysiological techniques. The questions that will be addressed are as follows: (1) What is the distribution of acetylcholine receptors in the heart? (2) Where is transmitter released and what is the relationship between transmitter release sites and receptor distribution? (3) How do the receptors respond to transmitter? (4) What are the mechanisms producing the slow synaptic response? (5) Is the number or distribution of receptors controlled by the presynaptic nerve in the heart? The answers of these questions should expand our understanding of synaptic transmission.